This invention relates to a method of determining the presence and location of seismic reflections from the earth's subsurface formations.
In seismic exploration, seismic energy is generated at a shotpoint at or near the surface of the earth, is reflected from subsurface interfaces between layers of the earth, and is received by a spread of geophones on the surface of the earth. The geophone signals are recorded in the form of a seismic record section. This seismic time section contains information which can be used to represent the charateristics of the subsurface formations.
Primary reflection signals on the seismic record section indicate the presence of a subsurface reflecting interface, and time occurances of these primary reflection signals represent the depth of the reflecting interface from the earth's surface. Furthermore, the time-shift or attitude of a primary reflection from trace-to-trace indicates the dip or slope of the subsurface interface.
Skilled seismic interpreters can sometimes determine the location and dip of subsurface interfaces directly from the seismic record section. However, this requires a large amount of judgment due to various distortions introduced into the record section. Such distortions may take the form of multiples, ghosts, reverberations, ground roll, shot noise, etc. Another type of distortion is from the physical phenomena, commonly known as migration. Such migration distortion is introduced by assuming that the reflection events come from a subsurface interface directly below the receiver and have been displayed on the record section as though this were the case. The impedance discontinuity producing the reflection is commonly termed the "reflector surface" and its apparent orientation on a record section is commonly termed the "record surface". The two surfaces as illustrated in FIGS. 1 and 2 will coincide only when the reflector surface is flat and horizontal. Migration is, therefore, a process by which the reflector surface is constructed on the record section as illustrated in FIG. 3. This is carried out by picturing the reflection points A', B' and C' as being moved to an arc to A", B" and C" directly under the shotpoints A, B and C.
In a typical seismic data processing sequence, migration is preceded by conventional common-depth-point stacking. Even though a large fraction of data is processed in this way, there are data sets where the depth-point stack does not enhance all the reflecting events of interest. In areas of complex geometry, criss-cross events may not stack well. Also fault plane reflections are lost during the common-depth-point stack. Diffractions, which help delineate the termination of reflectors, are generaly not preserved during the common-depth-point stack. Further, conventional migration techniques are desinged to operate in a homogeneous medium and are not satisfactory for continuing a seismic wave field in a medium with lateral velocity variations.
It is therefore, an object of the present invention to apply a prestack migration operation in such a manner as to produce a migrated seismic record section which overcomes the difficulties inherent in conventional migration processing.